Connector and circuit arrangement for automatic telecommunication exchange system



B. B. JOHNSON CONNECTOR AND CIRCUIT ARRANGEMENT FOR AUTOMATIC July 1, 1958 TELECOMMUNICATION EXCHANGE SYSTEM 2 Sheets-Sheet 1 Filed Oct. 16, 1956 H w a N. T? url a u H h q .OE N m :5 u m m5 i H 2: A v a H H E 9 $3 r H 0 m "5 1v at H QE 2. z 8 a News? 5 T N3 s a 3 s n I m2. Q A 0 IT... 3. w m .wWNY 05 2 2R v .2 N M 2 E? \l i: OCH :l mh iL (U 8 n Lwfi Ohm n. u mmm .mu 2.. M H3 um H m5 4 or. =8 1 mm: A! {m5 mmU b: WC. a3 95 m: a; U3 W iufi 22. q ww w r a m w mzok Sne /2 r I. u SJ $3 3 m: $3. NE 1 J 464 "I! m5 5 5 5 2 .m

y 1, 1953 B. B. JOHNSON 2,841,652

CONNECTOR AND CIRCUIT ARRANGEMENT FOR AUTOMATIC TELECOMMUNICATION EXCHANGE SYSTEM Filed Oct. 16, 1956 2 Sheets-Sheet 2 FIGZ calm

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. Z,8 .il,fi5Z Patented July 1, 1958 CONNECTOR AND CFLRCUIT ARRANGEMENT FOR AUTOMATIC TELECONIMUNICATION EXCHANGE SYSTEM Benjamin Bertie Johnson, West Dulwich, London, England, assignor to Telephone Manufacturing Qornpany Limited, London, England, a British company Application October 16, 1956, Serial No. 616,243

Claims priority, application Great Britain October 25, 1955 2 Claims. (Cl. 179-48) This invention relates to connector and circuit arrangement for automatic telecommunication exchange system.

Electric connecting devices responsive to impulses are well known in the telephone and other arts for counting a number of impulses and for performing or causing the performance of desired operations in dependence upon the number of impulses counted. They are mainly of one or other of two kinds, namely a step-by-step electromechanical switches or of relay chains. In the case of an automatic telecommunication system, the desired operation is the connection of a line (such as a trunk line) to a desired one of a plurality of subscribers lines, as a called line, whose identity is determined by the number of impulses applied to the connecting device from a calling telephone station.

The present invention is concerned with an electric connecting device responsive to impulses, which partakes more of the nature of a relay selecting chain than of a step-bystep electro-mechanical switch, and makes use of so-called selective relay units to perform the operation of selection in response to impulses and to effect connection to the wanted subscribers line.

The term selective relay unit as used herein denotes a device which consists of an electro-magnetic relay having a plurality of sets of contacts, a finger magnet associated with each set of contacts, each finger magnet, when energised, servin to interpose a finger between its associated set of contacts, or a card operating thereon, and the armature of the relay so that only the set of contacts associated with the energised finger magnet is operated upon when the armature of the relay is actuated, and an operating magnet for actuating the armature.

In our United States Patent No. 2,672,599, we have claimed an electric selecting device responsive to impulses from a source of impulses, wherein said device includes two selective relay units each having a plurality of sets of contacts, an armature, and an energising coil, characterised in that each relay unit is provided with a finger for interposition between each set of contacts and said armature, a finger magnet associated with each finger for interposing when energised its associated finger between a set of contacts and said armature and further characterised in that successive sets of contacts on either selective relay unit are connected to successive finger magnets on the other so that each selective relay unit, when it is energised closes an energising circuit for a finger magnet on the other, and means are provided for operating said relay units in alternation.

The device claimed in the said patent specification is capable of selecting and connecting a calling line to any one of ten wanted lines in response to the counting of an appropriate number of ten impulses in a single train.

The present invention which is an improvement in the invention claimed in the said United States patent specification provides a connecting device, responsive to a maximum of ten impulses per train of impulses but is capable of responding to more than a single train and in which is an electric selecting device comprising two selective relay units whose operating magnets are energised in alternation in response to successive impulses, and in which each selective relay unit when operated completes a circuit for a finger magnet of the other, thereby to produce a progressive selection of work con tacts of the device, characterised in that means are provided responsive to the reception by the device of a first train of impulses to select one or other of a plurality of groups of Work contacts and to render the device capable of responding to a second train of impulses whereby a particular work contact in the selected group of work contacts is selectable in response to said second train of impulses.

In our United States Patent No. 2,672,599, it is shown that selection of the desired group of work contacts may be efiected in one of three ways, that is- (a) By arranging the work contacts on each selective relay unit in two levels and, depending on the number of impulses in the first train, selecting one or other of the levels, or

(b) By providing additional pairs of selective relay units and, depending on'the number of impulses in the first train, selecting one or other of the pairs of selective relay units for response to the second train of impulses, or

(c) By providing additional selective relay units and, depending on the number of impulses in the first train, grouping the selective relay units in combinations of two for response to the second train of impulses.

According to a first embodiment of the present invention an electric connecting device for connecting an in coming trunk line to any one of a plurality of subscribers lines arranged in two groups the lines of one group having single-digit numbers and those in the other group having two-digit numbers said device including a pair of selective relay units arranged and adapted to operate according to the invention contained in United States Patent 2,672,599 is characterised in that each selective relay unit is provided with two levels of work contacts and in that a line in one said level is selected in response to a single train of impulses, said connecting device being provided with means responsive to a first train of impulses of a predetermined number of impulses for selecting the other said level of work contacts and with means for rendering said pair or" selective relay units responsive to a second train of impulses for the selection of a desired set of work contacts in said other level.

According to a second embodiment of the present invention an electric connecting device for connecting an incoming trunk line to any one of a plurality of lines arranged in groups, the lines in one of said groups having single-digit numbers and those in the other groups having two-digit numbers said device including a pair of selective relay units arranged and adapted to operate accord ing to the invention contained in United States Patent No. 2,672,599 is characterised by the provision of a third selective relay unit all said selective relay units having two levels of work contacts, and of means operable in response to a first train of impulses for setting said third selective relay unit in accordance with the number of impulses of said first train and received by said pair of selective relay units, and of further means responsive to said train of impulses for selecting one or other of the two levels of work contacts on said third selective relay unit in dependence upon whether the number of impulses in said first train is odd or even, and in that said third selective relay unit is provided with means for causing said pair of selective relays to respond to a second train of impulses it said first train comprises one or other of two predetermined numbers of impulses, and to select one or other of the levels of work contacts on said pair of selective relay units according to which predetermined number of impulses is comprised in said second train.

The invention is illustrated in the accompanying drawings, of which:

Figure 1 illustrates a connector and circuit for eighteen lines;

Figure 2 illustrates a connector and circuit for twentyeight lines.

in carrying the invention into effect, a connector comprises at least two cross-bar bridges, otherwise known as selective relay units; cross-bar bridges in consequence of their resemblance to the bridges, so-called, of a crossbar or coordinate selector, or selective relay units in consequence of their being, in effect, relays whose armature is incapable of actuating the banks of contacts until a bank is selected by a finger magnet adapted to interpose a finger between the armature and the wanted bank, or between the armature and a card by which the contacts of the wanted bank are actuated. Finger magnets are provided for each selective relay, and not common to a plurality of such as in the cross-bar switches.

The connector of Figure 1 involves two selective relay units and its circuit so organised as regards its wiring, that by the addition of a third selective relay unit and minor alterations to the wiring, the connector can be converted to that of Figure 2.

In Figure 1, the connections which need to be altered in order to convert the connector to that shown in Figure 2 are shown in broken lines and the connections that replace them, as well as the additional connections, are shown in Figure 2 in dot and dash lines.

The eighteen lines, selectable by the connector of Fig ure 1, are arranged in two decades, the first decade comprising the nine lines numbered 1 to 9, and the second decade comprising the nine lines numbered 01 to 09.

The two selective relay units of Figure 1 are referenced SRA and SRB respectively. The five odd-numbered lines in both decades are selectable by operation of banks of contacts of unit SRA and the even numbers, excluding and 00, in both decades are selectable by the operation of banks of contacts in unit SRB. Each column of contacts accommodates the negative, positive, and holding or test conductors of two lines, one in each decade. Each bank of contacts comprises eight levels of contacts six of which are required for the said negative, positive, and holding or test conductors of two lines. Differentiation between two decades is effected by operation of contact ch3, ch4, and ch6 of a relay CH. I

In describing the operation of the circuit of Figure 1, a connection to a line in the first decade will first be considered, and then a connection to a line in the second decade will be considered.

Connection to line in first decade When a calling line, not shown, desires a connection to, say line 5, his line first becomes connected to the connector of Figure l by a preselector, not shown, and presents the following conditions to the incoming conductors of the connector, that is (1) A loop to relay A,

(2) Negative, via a holding relay (not shown) in the preselector, to holding conductor H, and

(3) An earth to test conductor T. This is the conductor via which the preselector tested whether the connector was already engaged or was free.

Relay A operates in the loop presented to it by the calling subscriber, and relay CD operates in response to the earth on conductor T.

Relay CD, at contacts cdl returns earth via conductor H to hold the preselector, at contacts cd3 prepares a circuit for relay RF. Contacts cd2 are of no interest for the purpose of the present description. Contacts cd4 return dialling tone to the calling line.

Relay A, at contacts a1 closes a circuit for retarded relay B.

Relay B, at contacts b1 returns a substitute earth via conductor H to hold the prese'lector (not shown); at contact b3 provides a holding earth for relay CD, prepares a circuit (at present open at contact a2) for finger magnet FMAI of selective relay unit SRA and an energising cir cuit for relay RF, at b4 closes a circuit for starting a ringing and tone generator (not shown). Contacts b2 are of no present interest, and b5 is not used and therefore not shown.

Dialling tone is returned to the calling subscriber via front contact cd4 and back contact h5.

The calling subscriber may now pull and release his dial Upon release, the dial returns to normal and in doing so interrupts the loop circuit provided for relay A a number of times, in the chosen example five times. (For the present description, an impulse will be con sidered as on and oif respectively when the loop circuit is interrupted and restored.)

First impulse on, relay A releases and, at contact a2 closes the already-prepared circuit for finger magnet FMAl, although contact a1 opens the circuit of relay B, this relay being a retarded release relay does not release during the short on interval of this or any impulse. Finger magnet FMAI interposes finger fal between the armature AA of selective relay unit SRA and the card indicated by reference CA1 and at contact a1 closes a circuit for the operating electro-rnagnet EMA of selective relay unit SRA. Finger magnet FMAI interposed finger fal between the armature AA of selective relay unit SRA and the card, represented by line CA1, by which the first of left-hand bank of contacts can be actuated upon the energisation of EMA.

Selective relay unit SRA, has a number of (off-normal) contacts which are actuated, without the interposition of a finger, each time its electro-magnet (EMA) is energised. (Unit SRB is provided with similar off-normal contacts). Those of EMA are given references such as sml. Contact sral, sra2, sraf) have no immediate effect, contact sra4 opens the holding circuit (winding I) of retarded release relay CD, and connects earth to the upper terminal of winding I of relay SW which already has earth, from contact al at its lower terminal and therefore does not yet operate.

First impulse off (relay A re-energised). Contact a1 removes earth from the lower terminal of winding I of relay SW, which operates in series with electro-magnet EMA, the latter holding in this circuit, and contact 412 opens the operating circuit of finger magnet FMAIi. The finger fal is, however, held gripped between card, CA1 of the first bank of contacts, and armature AA by the latter. All the contacts in this left-hand column are actuated, but the only one of interest is that in level LAG. This will be referred to in connection with the second impulse.

When armature AA operates, a circuit is closed for finger magnet FMBl, via contacts rf6 (already operated) and srb3 not yet operated.

Relay SW, when it operates, at contact and connects its second Winding II to the contacts all (at present open) but is, itself still holding in its operating circuit, via its winding I.

Second impulse on relay A released. Relay A, at contact a1, closes a holding circuit for relay SW (winding II), and an operating circuit for electro-magnet EMB which is the operating magnet for the armature AB of selective relay unit SRB.

Electro-magnet EMB, when it energizes, at contact srbZ, opens the holding circuit of electro-magnet EMA via winding I of relay SW. Contact srb3 opens the circuit of finger magnet FMBl, but finger fbil, actuated thereby, is held gripped between card CB1 and armature AB, and contact srb4 closes a holding circuit via contact swZ, for EMB.

answers When EMA releases, an alternative holding circuit for EMB is closed at contact sra2, an operating circuit is closed for finger magnet FMA" from earth at contact 75, contact SW13, and contact 2 in level LBG of unit SRB.

Second impulse ofi (relay A operated). The only effects of this operation are to give relay B a further pulse, and to release relay SW (winding II).

Third impulse on (relay A deenergised). Contact at closes a circuit for EMA via SW1, which attracts its armature and, since FMAZ is energised, and finger fail; is actuated, the second column of contacts of unit SRA is actuated. In addition, EMA, at contact sraZ opens the holding circuit of EMB, which releases and at contact n53 closes a circuit for finger magnet FMBZ which interposes finger 1%.; between armature AB and the appropriate card, CB2. The operation of contact srad is unable to energise relay SW since its winding I is again short circuited.

Third impulse ofi (relay A reenergised). The shortcircuit is removed from winding I of relay SW, which operates.

Fourth impulse on" (relay A deenergiaed). The effect of this is the same as for the second impulse, except that finger fb2 having been actuated (see above) the second column of contacts in BBB is actuated by card CB2.

Fourth impulse oft (relay A reenergised). The effect of this is the same as for the third impulse.

Fifth impulse on (relay A deenergised). The eifect of this is the same as for the third impulse, except that, finger magnet FMA3 having been operated, the third column or contacts in unit SRA become actuated.

Subsequent impulses, i. e., the sixth, seventh, eighth, and ninth cause progressive actuation of the finger magnets in units SRA and SRB, with actuation of the banks of contacts in the units in alternation. These operations will not be further described.

The third column of contacts in unit SRA are those appropriate to subscriber 5, whose number it was assumed the caller wanted. This column is also appropriate to subscriber 05, but we are not yet concerned with this as the caller has transmitted five impulses and no further impulses follow, the loop is maintained closed and relay A remains operated.

Relay A, remaining operated, at contact all maintains relay B operated, and holds off the earth from winding 11 of relay CD. The circuit for winding I of CD is open at contact mad, and relay CD ultimately releases, and at contact cdl 'emoves its earth from conductor H: contact b1, however, provides an alternative earth for this conductor. Contact cdZ is of no interest, contact cd3 opens the circuit of relay RF (which releases), and contact cd-t performs a preparatory operation which will be noticed later.

Conditions are now, that the called line 5 is tested for busy, and if idle is rung.

Busy test The busy test is eflected as follows: the wanted line (not shown), it free, presents to the connector a negative potential on its test conductor 5.

Ringing As soon, therefore, as relay CD releases, relay H opcrates if line 5 is free (earth at contact cdl, winding I of relay H, contact 0116 at rest, the third contact in the said level LAE the said cut-01f relay (not shown) of the called line, to negative.

Relay H operates in this circuit and immediately closes its x contact hll, whereby relay H, if it has not already done so, fully operates, via its winding II and contact fi at rest. Contacts hit and 113 close points in the connecting circuit, whereby, inter alia ringing is interruptedly applied via interrupter contacts ta to the called line, the return being to earth at contact hS, via contact f3, at rest.

Between periods of application of interrupted ringing to the wanted line, Winding I of relay F is connected thereto. Relay F will not operate so long as the wanted subscriber has his telephone on the cradle.

However, when the called subscriber answers, relay F operates as soon as its winding I is applied to the called line and, at x-contact f1 removes a short circuit from its winding II. If relay F has not already fully operated, it now does so in a circuit which includes its; own winding 11 and winding ii of relay H.

Contacts f2 opens the circuit of the tone and ringing generator (not shown), f3 and f4 open the ringing circuit; also closes the connector circuit towards the calling line.

(Note the current feed for the calling and called lines is effected by relay A.)

Release of the established connection is etiected, when both the subscribers hang up their telephone, by the release of relay A, followed by the release of relay B and consequential release of the connector and removal of earth from conductor H and from the conductor t5 of the called line when the connector releases. Had the called line been already engaged relay H would not have operated by reason of there being earth already connected to conductor t5.

Calling a 2-digit number it may have been noted that all odd lines 1-9 are are reached over levels LAA and LAC of selective relay SRA, and that even numbers 2-8 are reached over levels LBA and LBC of selective relay unit SRB. These are one-digit numbers.

If one of the odd numbers 01-09 is wanted, it is reached over levels LAB and LAD of unit SRA the even two-digit numbers being reached over levels LBB and LBD of unit SRB. There are two-digit numbers and are distinguished by initial digit 0 which, consequently, does not appear as a one-digit number.

It is thought to be unnecessary to' describe, step-bystep, the operations resulting from the transmission of the impulses, for digit 0.

The conditions at the ninth impulse off are, that finger magnet FMBS is energised and finger ,fb5 is interposed between card CBS and armature AB.

When the tenth impulse is on" (relay A released), a circuit is completed for electro-magnet EMB, whereby finger fbS is gripped between card CBS and armature AB. The fifth bank of contacts in SRB are thereby actuated. EMA is released.

In this condition, a circuit is closed for relay CH. This circuit is from earth at contact rf6, via. contact sra3, contact 0 in line LBG of circuit SRB.

Relay CH, at contacts c123, ch4, and chd switches the connector circuit from levels LAA, LAC and LAE of units SBA and SRB, to levels LAB, LAD and LAF respectively. Contacts chi and chS being permanently short circuited serve no purpose in this circuit, and com tact ch2 closes a holding circuit for relay CH, which depends, alternately on contacts sral or srbl as energising magnets EMA and EMB operate in alternation in response to the second train of impulses.

Relay CD is held operated, in a circuit from earth, via contact 0 level LHB of SRB, terminals T218 and TY], winding I of relay CD, to negative.

Selection of any number 1 to 9 in the second decade 0, is eftected in the manner already described for a onedigit number.

Converting Figure 1 to a 3-decade system The two-digit, eighteen-line connector circuit of Fig ure 1 may readily be converted to a twodigit twentyeight-line connector of Figure 2. For this purpose, the following connections have to be removed: that is, the dotted-line connections, between, on the one hand, terminals TX]. to TXll and on the other hand TYl to 7 TX11 and that between terminal TY7 and TZih, and TY13 and earth.

A third selective relay unit having finger magnets and fingers, an operating electro-magnet, an armature, five banks of contacts, and otf-norrnal contacts is introduced to the circuit. The five finger magnets FMC1- ViC (see Figure 2) of unit SRC are connected to terminals TZ14 to TZ18, which are the terminals for the five parallel-connected contacts in level LAH and LBI-I of selective relay units SRA and SRB.

The operating magnet EMC is connected to terminal TZ8.

Contacts src4 are connected to terminals TXS, TY8, and TZ8, as shown.

Terminals TX1 to TX4 and TX6 are respectively connected to the commoned-contacts in levels LCA to LCD and LCF of SRC.

Terminal TXS is connected to terminal TYli and terminal TY11' is connected to terminal TX11 via contacts sral. The effect of this is to remove contact sral from the position it occupied in the circuit of Figure 1 to a position between back contact 0116 and the commoned contacts in level LCE of selective relay unit SRC.

Terminals TX10 and TY11 are connected to terminal T210 and TZll respectively. The effect of this is to bring contact srcl between earth and contact ch; so as to replace contacts sral and srb1 which, in parallel, originally occupied this position. Contact srbl is left disconnected.

Terminals TXlZ and TX13 are connected to terminals TY12 and TY13 respectively so that the first digit, when dialled, can be transferred to the third selective relay unit SRC, as will be described.

Terminals TYl and TY2 are connected to the two contacts C8 and C0 in level LCB. Terminals TY3 and TY t, and TY5 and TY6 are connected to corresponding contacts in levels LCD and LCF respectively. Terminal TY7 is connected to contacts G8 and G0 in level LCC of SRC.

Operation of the thus modified circuit will now be described with reference to Figure 2.

A single digit number 1 to 6, or 7 or 9, is selected in exactly the same manner as would be the case with the circuit of Figure 1, that is by progressive energisation of the finger magnets FMA and FMB and energisation in alternation of electro magnets EMA and EMB of selective relay units SRA and SRB.

Connection to the called line, if it has a one-digit numher, is, however, effected, not as in Figure 1 via contacts of units SRA or SRB, but via the appropriate bank of contacts of the added selective relay unit SRC.

, The manner of effecting this is as follows:

Assume that, as before, line number 5 is being called and recall that when the fifth impulse is off, EMA is energised and EMB is not energised, and that the contacts in the third column of all the levels, including level LAH, of selective relay unit SRA are closed, and thatultimately, relay CD releases. 7 Consequently, finger magnet FMC3 energises, in a circuit from earth, the circuit being, earth via contacts rf6 (front), srb3 (back), terminals TX11. and TYlZ, the third contact in level LAH of selective relay unit SRA, and finger magnet FMC of selective relay unit SRC. This finger magnet interposes finger 103 between card CC3 and armature AC. There being no further impulse or train of impulses, relay A remains operated and opens any possible circuit for winding 1 of relay CD.

The circuit for winding I of relay CD is opened at contact sra i (electro-magnet EMA being energised).

Relay CD releases. Electro-rnagnet EMC for unit SRC is energised in a circuit from earth, via contacts 123 (front), srcd (back), sra2 (front), sdZ (back), the winding of EMC to negative and at contact src4 closes a holding circuit for itself.

. Contact cdS releases relay RF which at contact rf6 releases finger magnet FMC3. The contacts in the third 3 column of SRC, however, remain closed, because finger fc3 is held gripped between card CC3 and armature AC3.

The connection between the calling trunk line and wanted subscribers line 5 is now. effected via back contacts c113 and chi, and the fifth contact in levels LCA and LCC of selective relay unit SRC.

Ringing of the called line, or the giving of busy tone to the calling line are efiected as described in connection with Figure 1.

Had the single digit been even, say 4, the selecting operation would have been effected as described in connection with Figure 1. When the 4th impulse is off, electromagnet EMB remains operated and EMA is re leased.

In this case, finger magnet FMC2 would be operated in a circuit from earth at contact rf6, sra3, terminals TXL: and TY13, contact 2 of level LBH.

Relay CD releases, after a delay, as its circuit is opened at contact srb2 (back). As a result electro-magnet EMC operates in a circuit from earth at contact 123, via front contact .s'rbZ, sdZ (back).

As line 4 is in levels LCB and LCD it is necessary to operate relay CH. This is eflected as follows:

After (as already described for a call to line 5) relay CD released, it operated again in the following circuit: earth at contact b3 via back contact srbZ (electro-magnet EMB having been released, by the opening of contact src4), back contact sra4, and back contact chS.

Relay CD at contact cd2 closes an operating circuit for relay CH from earth at contact b3, via operated contacts src l, and 0:12 and back contacts ch2. Relay CH holds via front contacts ch6 and srcl. Relay CH, at contact 0125', opens the circuit for winding II of relay CD, which again releases to test the wanted line for busy or free condition, as for the previous connection.

Again, ringing of the wanted line or busy tone to the calling line, as circumstances may require, are as before.

Had the call been for one of the two-digit numbers in the eight or naught decade, the calling subscriber would have dialled, as a first-digit, one of the numbers 8 or 0, and these would have been transferred to selective relay unit SRC in just the same way as digit 4 was so transferred, and being even numbers, it would have required the operation of relay CH.

Relay CH would have operated in the manner described for single-digit number 4.

When the calling subscriber transmitted the first impulse of the second digit, relay A would have released and closed, at contact :12, a circuit for finger magnet FMAll. This can be traced from earth at contact b3, via back contacts srbZ, sra4, contact 8 or 0 in level LCG of selective relay unit SRC (instead of via contact 0125, which is now open), and released contact a2.

Procedure from this point is, as already described, by progressive energisation of finger magnets FMAl to 5 and FMBl to 5 and alternate energisation of electromagnets EMA and EMB.

It is thought not to be necessary to describe these further operations as such further description would be mainly repetitive.

What is claimed is:

1. An electric selecting device, for connecting, in response to impulses from a source of impulses, a calling line to a desired one of a plurality of other lines divided into a first and a second group, said device in cluding a pair of selective relay units, an impulse-switching relay, and a change-over relay; each selective relay unit having an armature, an energising coil for operating said armature, a plurality of banks of contacts, a like plurality of fingers and a like plurality of finger magnets, one finger and one finger magnet associated with each bank of contacts, each bank of contacts including at least two work contacts, to each of which work contacts one line of said plurality of said other lines is connected, and a control contact, connections connecting each control contact of one selective relay unit to a finger magnet of the other selective relay unit, said impulse-switching relay having contacts for distributing impulses from said source of impulses in alternation to said energising coil of said selective relay units so as to produce a successive operation of said banks of contacts in response to successive impulses of a first train of impulses, circuit means including a controlling contact of one of said selective relay units for operating said changeover relay in response to a given impulse of said first train of impulses for causing said two selective relay units in response to a second train of impulses to select a line of the second group, said change-over relay when operated cutting out the lines of said first group of lines.

2. An electric selecting device, for connecting, in response to impulses from a source of impulses, a calling line to a desired one of a plurality of other lines divided into a first, a second, and a third group, said device including a pair of selective relay units, a third selective relay unit, an impulse-switching relay, and a changeover relay, each selective relay unit having an armature, an energising coil for operating said armature, a plurality of banks of contacts, a like plurality of fingers, and a like plurality of finger magnets, one finger and one finger magnet associated with each bank of contacts, each bank of contacts including at least two work contacts to each of which work contacts one line of said plurality of said other lines is connected, each bank of contacts of said pair of selective relay units: also including a control contact and a transfer contact, connections connecting each control contact of one selective relay unit of said pair to a finger magnet of the other selective relay unit of said pair, said impulse-switching relay having contacts for distributing impulses from, said source of impulses in alternation to said energising coil of said selective relay units so as to produce a successive operation of the banks of contacts of the selective relay units, of said pair of selective relay units llll response to successive impulses of a first train of impulses, connections connecting each transfer contact of both selective relay units of said pair to a finger magnet of said third selective relay unit, circuit means including auxiliary contacts of said pair of selective relay units for operating said change-over relay in response to a given impulse of said first train of impulses for causing said third selective relay unit in response to a second train of impulses to select the second or third group of lines.

No references cited. 

